Enzymatic Carbon−Carbon Bond Formation in Water-in-Oil Highly Concentrated Emulsions (Gel Emulsions)

Water-in-oil (W/O) highly concentrated emulsions (gel emulsions) of water/C 14 E 4 /aliphatic hydrocarbon systems were investigated as reaction media for the aldolic condensation of dihydroxyacetone phosphate (DHAP) with acceptor aldehydes such as phenylacetaldehyde ( 1 ) and benzyloxyacetaldehyde ( 2 ), catalyzed by d -fructose-1,6-bisphosphate aldolase from rabbit muscle (RAMA). Prior to any enzymatic reaction, both the formation and stability of the W/O gel emulsions in the presence of reactants were assessed. It was found that the aldehydes improved greatly the kinetic stability of W/O gel emulsions at 25 °C by decreasing the hydrophile−lipophile balance temperature ( T HLB ) of the water/C 14 E 4 /aliphatic hydrocarbon systems. Interestingly, the stability of RAMA in W/O gel emulsions was improved by 7- and 25-fold compared to that in aqueous medium or conventional dimethylformamide/water 1/4 v/v mixture, respectively. It was found that the equilibrium yields and enzymatic activity depended on both the aldehyde partitioning between the continuous and dispersed phases and the water−oil interfacial tension. The highest enzymatic activities were achieved in W/O gel emulsion systems with the lowest water−oil interfacial tension. The equilibrium yield depended on the water−oil interfacial tension for the hydrophobic phenylacetaldehyde, and on the partition coefficient for the hydrophilic benzyloxyacetaldehyde. Optimum equilibrium product yields (65−70%) were achieved at either the lowest water−oil interfacial tension or partition coefficient values.